1. Field of the Invention
The present invention relates to self-supporting dynamic polymer membranes (“dynamer” membranes) of polyimine type, to a process for preparing them and to their use in separation processes, especially for gaseous species.
2. Description of Related Art
Various types of process exist for separating chemical species, such as cryogenics, distillation, solvent absorption (chemical or physical), adsorption and membrane separation.
In the field of separation of gaseous species, in particular in the treatment of natural or synthetic gases, the separation and purification of the components is essential in order to satisfy the increasing needs of the users. Thus, crude natural gas and the derived components must be freed, inter alia, of the carbon dioxide contained by means of an operation known as deacidification.
Among the various separation processes known in the prior art, membrane separation is the least energy intensive and is among the processes that are the most widely used at the industrial level.
Among the membrane processes, the following are distinguished:                processes using microporous inorganic membranes essentially composed of alumina, silica, zeolites or carbon. These processes are efficient, tolerate moderate flows and also high temperatures and pressures, but are expensive. Furthermore, the selectivity factors of inorganic membranes with respect to the chemical species to be separated remain moderate;        processes using polymeric membranes are less expensive in raw material and energy terms. However, the membranes used degrade quickly. The development of organic membranes and their uses for gas separation have been envisioned with the aid of polymers of very varied structures. The majority of the processes industrially used involve membranes manufactured as vitreous polymers, for instance, polyimides, polysulfones and polyphenylene oxides since they generally have greater selectivity and better mechanical properties, but, in counterpart, they accept a smaller separation flow than inorganic membranes since they have insufficient permeability. Other polymers, for instance elastomers such as polysiloxanes, for example, have also been used. They have permeability higher than that of membranes manufactured from vitreous polymers, but are less selective toward the gaseous species to be separated (A. Stern, J. of Membr. Sci., 1994, 94; S. T. Hwang et al., Separation Science, 1974, 9(6)). In general, it has been found that there is an inverse relationship between selectivity and permeability: the better the selectivity, the lower the permeability. Thus, even though membrane processes represent an advance compared with more standard processes, they still need to be improved especially because the membranes used remain expensive and because it is often necessary to make a choice between a high flow (high permeability) and high selectivity.        
The selectivity of the polymeric membranes used toward the chemical species to be separated is, moreover, modulable only if the chemical nature and/or the content of the monomers constituting them are varied. Membranes for the selective separation of gases, formed from a copolymer of ethylene oxide (EO) and of epichlorohydrin (EP) have thus already been proposed, in particular in patent application EP 1 322 409, some of the ethylene oxide units possibly being replaced with propylene oxide (PO) units. These membranes are useful in particular for the selective separation of the carbon dioxide (CO2) contained in a gaseous mixture. The best selectivity toward CO2 is obtained with a membrane formed from EO/EP/PO units in proportions of 85/2/13 (mol %). The article by Sanchez J. et al. (Membrane Science, 2002, 205, 259-263) relates a study of the permeability of self-supporting films obtained from crosslinked copolymers of poly(ethylene oxide) (PEO) and of epichlorohydrin. It is indicated therein that it is possible to vary the CO2 permeability properties by varying the PEO/epichlorohydrin (PEO/EP) ratio. The best results are obtained with copolymers containing between 87% and 96% of ethylene oxide units, the maximum CO2 permeability being obtained with a content of 93%.